The present invention relates to pressure sensing transducers of the type utilizing strain sensitive elements formed in a flexible diaphragm subjected to a pressure to be measured. More particularly, the present invention relates to an improved pressure sensing assembly mounting arrangement with a hermetic seal of the high pressure chamber for reduced component stress, media compatibility, improved stability and improved measurement accuracy at elevated temperature.
Semiconductor pressure transducers have a wide range of applications including industrial and other applications where accurate pressure monitoring is required especially under harsh environments. Semiconductor pressure transducers utilizing silicon, sapphire or other crystalline diaphragms offer many potential advantages in such applications due to their small size, absence of moving parts and potential for sensitivity and accuracy.
The typical semiconductor pressure transducer basically consists of a pressure force collector diaphragm having one or more electrical strain sensitive elements such as piezoresistors or a capacitor mounted thereon which change electrical characteristics with the deflection of the diaphragm where such changes are detected, amplified and relayed to various instrumentation which indicates the pressure history of the monitored system.
In one prior art approach, dopant silicon piezoresistive elements are formed directly in a force collector diaphragm of single crystal silicon. Since the silicon piezoresistive film is integral to the silicon diaphragm, the piezoresistive film is essentially an atomic extension of the diaphragm and has the same crystal structure. This results in improved bonding and effectively no hysteresis effect. Additionally, the piezoresistive elements may be formed in specific orientations according to the needs of the particular transducer. Specifically, a Wheatstone bridge configuration of silicon piezoresistive elements may be laid out on the diaphragm using techniques well known in the art such as doping, masking and etching.
Although having many advantages, such silicon transducers also have inherent disadvantages as well, particularly at elevated temperatures. Since the silicon diaphragm is a semiconductor by nature, electrical leakage between the piezoresistive elements through the silicon diaphragm may occur at high temperatures. Each silicon piezoresistive element is typically formed in an island of oppositely doped conductivity type, where the P-N junction prevents current flow from the piezoresistive film into the diaphragm. However, at higher temperatures, typically those above 350.degree. F., the P-N junction typically experiences complete failure and/or undesirable electrical characteristics.
In order to overcome the problem of the breakdown of the P-N junction at high temperature between the piezoresistive elements and the diaphragm two approaches have been used. In the first, a sapphire material has been used as a diaphragm. This technique is described in U.S. Pat. No. 4,994,781 the disclosure of which is incorporated herein by reference. Since sapphire is an electrical insulator, there is no need for a reversed biased semiconductor junction between the piezoresistive element and the diaphragm. However, differences in the coefficient of the expansion of the different materials that are used in the piezoresistive elements, the force diaphragm, the support element, and the transducer housing can still combine to induce stress related failures in one or any of the parts especially at elevated temperature.
A second approach is to use a diaphragm assembly configuration referred to in the art as silicon-on-insulator. In this method, an insulating oxide layer is created between two layers of single crystal silicon. One of the layers of silicon is very thin while a second layer is relatively thick. The pressure sensitive diaphragm is formed in the thicker silicon layer while a plurality of strain gages are formed in the thinner layer using techniques well known in the art such as doping, masking or etching. The rest of this thin silicon layer is etched away, leaving the strain gages as dielectrically isolated islands on a silicon diaphragm.
To utilize silicon or sapphire diaphragms in pressure transducers, it is necessary to suitably mount the diaphragm in a housing adapted to be connected to a source of pressure to be measured. For many industrial and aerospace applications, the media, pressures and temperatures are so extreme that a rugged mounting and sealing arrangement for the pressure sensing diaphragm is required and heretofore has not been available.
In order to achieve adequate strength, the diaphragm is commonly mounted on a support structure. If the thermal expansion coefficients of the support are substantially different than the diaphragm assembly, temperature variations can cause stresses and strains to be produced in the diaphragm and support structure giving rise to stress induced failure and measurement error. This error arises because thermal stresses produced in the diaphragm cause changes in the electrical properties of the diaphragm strain sensing elements which are indistinguishable from those changes caused by pressure induced bending strain in the diaphragm. Selecting a support material that has a coefficient of thermal expansion that matches that of the diaphragm to minimize thermally induced stress is a solution to this problem and is known in the prior art. The problem with the prior art is specifically with hermetically sealing the diaphragm support to the housing for use at high temperatures and pressures.
One method to seal the diaphragm support element in a pressure transducer is disclosed in U.S. Pat. No. 3,697,917, the disclosure of which is incorporated by reference, which employs an elastomer to seal the pressure to be measured from the ambient pressure across the diaphragm. Elastomers do not function well at high temperatures and pressures. In addition, they may pose problems of compatibility for some media.
Another sealing method is disclosed in U.S. Pat. Nos. 4,918,992 and 4,019,388 where a glass support member is bonded and sealed to a metal housing by soldering. This sealing technique requires that the glass support member be coated with a solder wetable metal such as a mixture of titanium, platinum and gold. This process is complicated and expensive with an additional soldering operation necessary to complete the seal and in addition the seal does not function well at high pressure.
Due to the deficiencies of the prior art, a need presently exists for an improved type of pressure sensor that employs a proper selection of materials for mounting of a pressure sensitive silicon or other crystalline type diaphragm on a support element and for the hermetic mounting of the support element to seal the high pressure fluid to be measured from the reference pressure for operation at high temperatures and pressures and in corrosive environments while having a high degree of accuracy.